Boron carbide (B4C) is one of the hardest materials known, ranking third behind diamond and cubic boron nitride. It is the hardest material among mass-produced materials (i.e., materials produced in tonnage quantities).
Boron carbide may be used in a wide variety of applications, including ballistic and abrasive applications. For example, boron carbide is the Defense Department's material of choice for ballistic applications, such as body armor. Also, boron carbide materials may be used in military and commercial vehicles in war zones to protect against the pervasive threat of improvised explosive devices. Boron carbide materials may help improve survivability and mobility in future military combat vehicles and aircraft. Boron carbide materials, however, have an Achilles' heel in that conventional means of making boron carbide have several drawbacks.
Commercial production of boron carbide powder may be accomplished via several methods. Boron carbide powder may be produced by reacting carbon with boron oxide (B2O3) in an electric arc furnace at high temperatures, through Carbothermal reduction or by gas phase reactions. The process is strongly endothermic. The starting material may be an intimate mixture of boric oxide and petroleum coke. In addition to boron carbide powder, large amount of carbon monoxide are generated. For commercial use, boron carbide (B4C) powder usually needs to be milled and purified to remove metallic impurities.
Another production process of boron carbide powder is the reduction of boron with the presence of magnesium. This process is highly endothermic and typically occurs at 1,000-1,200° C.
Conventional boron carbide parts may be fabricated by hot pressing, sintering and sinter—Hot Isostatic Pressing (HIPing). Industrially, densification may be carried out by hot pressing (e.g., 2,100-2,200° C., 30-40 MPa) boron carbide powder in an inert atmosphere, such as argon. This commercial process (i.e., hot pressing) squeezes boron carbide powder together between large dies, while heating to elevated temperatures, and yields materials with a relative density that could be as high as 98.1% theoretical density. Other materials that may be used include: Al, V, Cr, Fe, Co, Ni, Cu, Mg, BN, MgO, Al2O3, etc. Hot pressing is typically used to manufacture simple shapes. Improved properties may be obtained when pure fine powder is densified with sintering aid additives.
Typical firing processes include two step heating cycles: binder burnout (or burn-off) and sintering. The first heating cycle, binder burnout, typically occurs at relatively low temperature (500-600° C.) and functions to remove the binder. This typically includes burnout or removal of the cellulose, which acts as a binder. For example, hydrogen and oxygen are removed and carbon monoxide is produced. As such, there is no, or very little, free carbon remaining. The second heating step includes sintering at a very, very high heating cycle (around 2,200° C.). Sintering basically fuses all the particles together to make a single solid part.
Modified boron carbide formation processes also exist. For example, a pressureless sintering process. This pressureless sintering process (e.g., 2,000-2,200° C.) may improve the density and hence the ballistic performance of boron carbide. Pressureless sintering to high density is possible using ultra fine powder with additives (e.g., in-situ carbon, alumina). The pressureless sintering process yields a 92-97% theoretical density (Dth).
For more demanding applications, post-sintering hot isostatic pressing (HIPing) may be used to increase the relative density to 99% (Dth) through the hydrostatic squeezing action at a high temperature and pressure (e.g., 1,700° C., 200 MPa), in a controlled atmosphere (e.g., Ar, He gas).
The boron carbide powder used to form conventional boron carbide materials has a reputation for poor performance during sintering—a high-temperature process in which particles consolidate, without melting, to eliminate pores between them in the solid state. Poor sintering yields a more porous material that fractures more easily. Due to the difficulties in sintering to high densities, metal carbide ceramics, such as boron carbide (B4C) ceramics, are very hard to manufacture.
These manufacturing difficulties multiply for the fabrication of fibers made of these materials. Exemplary difficulties that may be encounter in attempting to manufacture boron carbide fiber include: spinning and material compatibility with the fiber manufacturing process being employed; fiber carbonization and controlling chemical reactions; stoichiometric production of boron carbide and compositional control; sintering study to produce high density boron carbide fiber; scale up production; and the like.
Alternative laboratory routes also exist and include CVD, crystal growth, etc. These routes are typically high cost, low volume, and slow.
What is needed is a process that allows a fiber comprising carbon and metal-based materials to be formed. What are also needed are improved manufacturing processes to facilitate formation of an improved metal carbide fiber. For example, a metal carbide fiber having light weight, increased hardness, and improved ballistic and/or erosive performance—than currently available boron carbide products made from boron carbide powder. Further, what are needed are improved methods of making metal carbide fibers that are easier to form and yield higher production volumes at lower costs. Improved boron carbide fibers, fiber composites, and methods of manufacturing boron carbide fibers that solve more than one or all of the disadvantages existing in the prior art while providing other advantages over the prior art would represent an advancement in the art.